Aquaculture seems to be the only possibility to cover the growing demand of aquatic food that cannot be covered by extractive fishing. World aquaculture production has grown during the last fifty years from a production of less than 1 million tonnes in the early 1950s to 59.4 million tonnes by 2004. This level of production had a value of US$70.3 billion. A 69.6 percent of the world wide global production was produced in China, 21.9 in the rest of Asia and the Pacific region, 3.5 in Western European region and 0.4 in Central and Eastern European region. With regard to environment, in 2004 aquaculture production from mariculture (saltwater aquaculture) was 30.2 million tonnes, representing 50.9 percent of the global total. Freshwater aquaculture contributed 25.8 million tonnes (43.4 percent) and the remaining 3.4 million (5.7 percent) tonnes come from production in brackish environments. All the data are from Fisheries Technical Paper 2006, State of World Aquaculture (FAO).
Aquaculture production in Europe is only 3 percent of world production, but is leader in several species production, like atlantic salmon, trout, seabass, gilthead seabream, turbot and mussel. In 2002 production was higher than 1,3 million tonnes with a value of 3280 million. The most important species in Mediterranean countries are gilthead seabream, seabass and turbot. Production of both gilthead seabream and seabass in 2002 was 181000 tonnes and principal producers were Greece, Turkey, Spain, Italy and France. Production of turbot in 2002 was 5320 tonnes from which a 75 percent was produced in Spain. All the data are from FAO.
Fish disease has become a setback to aquaculture growth and is now responsible for the severe impact on economic development in many countries of the world. Diseases cause high mortalities in many cases and it is the origin of important production losses. Aquaculture losses also cause a reduction in food availability, loss of income and employment, with all the associated social consequences. The most usual diseases and their causal agents (bacteria or virus) affecting the above mentioned species are: vibriosis (Photobacterium damselae subsp. damselae, Vibrio spp.), pasteurellosis (Photobacterium damselae subp. piscicida), photobacteriosis (Photobacterium damselae subp. pasteurella), flexibacteriosis (Tenacibaculum maritimum), myxobacteriosis (Flexibacter maritimus), furunculosis (Aeromonas salmonicida), streptococcosis (Streptococcus parauberis), winter disease syndrome (Pseudomonas anguilliseptica), viral encephaloretinopathy (Nodavirus), lymphocystis (Iridoviridae), distended gut syndrome (virus-like particle), infective pancreatic necrosis (IPNV), infective haematopoietic necrosis (IHNV) or viral haemorrhagic septicaemia (VHS). All the data are from Cultured Aquatic Species Information Programme (FAO). Culture conditions of species in aquatic farmer, i.e. high densities of animals or aquatic environment, are suitable for infectious disease agents transmitted between individuals. Furthermore, the stress produced to the fish during their manipulation in the farmer could suppose a depression of immune system that facilitates infection by pathogens. Therefore, many efforts in research have been dedicated to development of vaccines or immunostimulants products to prevent diseases in fish. Prevention of diseases also reduce environmental impact because avoid massive use of antibiotics in aquaculture. Several products have been used for fish vaccination or immune system stimulation. Actually, there are vaccines available for usual bacterial fish diseases, like vibriosis, photobacteriosis, furunculosis, flexibacteriosis, winter disease syndrome or streptococcosis and also for usual viral fish diseases, like IPNV or IHNV (reviewed by Toranzo et al., 2005 and Sommerset et al., 2005).
There are three common methods of vaccination: immersion, injection and oral administration. Intraperitoneal injection is the most effective route for vaccination because it allows a better control of the dose and permits the use of adjuvant which results in better immune response. But there are disadvantages for injection route due to the fact that fish sedation and manipulation can cause a stress in animals and can origin damage to fish when vaccine is not administered with care. Moreover, small fish can not be vaccinated by this route. Immersion method is frequently used in farmer because is easy and fast but it does not allow a strict dosage control and is stressful for fish with consistent immune system depression. Oral administration of vaccines does not require fish manipulation and it is a suitable method for vaccination of all sizes animals, included juvenile fish that are starting to feed, which is very important because these fish are more susceptible to be infected by pathogens. Several disadvantages of oral route are that does not allow a strict dosage control and a large quantity of antigen is required for immunization. The most important obstacle for oral administration is that antigens are often inactivated in gut by acidic environment or protease activity and it prevent active antigens absorption by intestine. For this reason an effective oral administration requires the antigen protection, i.e: encapsulation, to avoid antigen degradation in gut (Hart et al., 1988; Quentel and Vigneulle, 1997). Different approaches to protect the antigen from degradation have demonstrated some promising results such as entrapping in liposomes or alginate microparticles (Ire et al., 2005; Maurice et al., 2004).